Polyamides, in particular nylon-6, are typically pelletized after the polymerization/polycondensation process, and extracted with water in order to remove residual monomers and oligomers, and then dried. Postcondensation can optionally be carried out after the extraction process in order to increase molecular weight and, respectively, viscosity. The moisture deriving from the extraction process is removed here from the polymer pellets. Various processes for this are known.
EP-B-1 235 671 relates to a continuous process for the drying and postcondensation of pellets, and also to a system for conduct of the process. The process decouples drying and postcondensation, and the drying process is carried out in a crossflow apparatus. The resultant predried polyamide is then postcondensed in a vertical duct apparatus in countercurrent with a slow stream of nitrogen.
EP-B-0 732 351 relates to a process and an apparatus for the production of polyamides, where polyamide pellets produced through polycondensation in the melt are postcondensed in the solid phase. Here, the polyamide pellets are heated in a first step to a first temperature of from 70° C. to 150° C., kept for from two to six hours at said temperature, and then, in a second step, heated at a temperature which is higher, but below the melting point, being from 170° C. to 10° C. below the melting point, and postcondensed at this temperature. The postcondensation can be carried out in a stream of inert gas. The postcondensation is preferably carried out in a moving-bed reactor to which inert gas having at least two different temperatures is introduced at least two different sites. By way of example, nylon-6 is postcondensed first at a temperature of 150° C. and then at a temperature of 190° C. The introduction of the inert gas at two sites serves to generate the different temperatures.